The present disclosure relates to current limiting circuits for boost converters which supply an output voltage higher than an input direct-current voltage.
The boost converter is a switching-mode power supply circuit which supplies an output voltage higher than an input voltage from an input power supply, such as a battery or the like. The boost converter includes an inductor one end of which is connected to the input power supply, a main switch which is connected to the other end of the inductor and applies an input voltage to the inductor during the on-state, a rectifier which is also connected to the other end of the inductor and flows a current to the output when the main switch is off, a smoothing capacitor which smoothes power obtained via the rectifier to supply an output direct-current voltage, and a control circuit which performs switching control to stabilize the output voltage while adjusting an on-time and an off-time of the main switch.
In the boost converter, when the load rapidly increases or when switching is performed so that the target output voltage is increased, the on-time of the main switch is increased so that the output voltage is controlled, and therefore, a current flowing through the inductor (hereinafter referred to as an inductor current) rapidly increases. In order to protect components, such as the main switch, the inductor and the like, from such an increase in the inductor current, a current limiting circuit is used to detect the inductor current and block the output so that the inductor current does not exceed a predetermined limit current value.
When there is a sufficient margin between the inductor current within the normal operating range and the limit current value, the current limiting circuit can be designed so that it does not have to have very high current detection accuracy, but its components need to have excessive performance for resistance to current. In order to reduce the margin between the inductor current and the limit current value to avoid this, it is necessary to detect the inductor current with high accuracy.
Japanese Patent Laid-Open Publication No. 2008-131764 describes a current limiting circuit which flows a constant current to a reference transistor of the same type as that of the main switch of the boost converter, and uses a comparator to compare a voltage between both ends of the reference transistor (a reference input to the comparator) with the drain voltage of the main switch (a comparative input to the comparator), thereby canceling initial variations in the voltage between the reference input and the comparative input of the comparator, and temperature fluctuations to increase the current detection accuracy.
FIG. 4 is a diagram showing a circuit configuration of a boost converter having a conventional current limiting circuit (see Japanese Patent Laid-Open Publication No. 2008-131764). In the boost converter of FIG. 4, the reference character 1 indicates an input direct-current power supply which outputs an input direct-current voltage Vin. The reference character 2 indicates a boost circuit which is a boost DC-DC converter which includes an inductor 21, a main switch 22, a rectifying diode 23, and a smoothing capacitor 24, and generates an output voltage Vo from the input direct-current voltage Vin. The reference character 10 indicates a control circuit which detects the output voltage Vo and outputs a signal which is used to switch the main switch 22 so that the output voltage Vo becomes a target value. When the main switch 22 is turned on by the switch signal, the input direct-current voltage Vin is applied to the inductor 21, and therefore, the inductor current increases while energy is accumulated in the inductor 21. When the main switch 22 is turned off, the inductor current flows via the rectifying diode 23 to the smoothing capacitor 24 to charge the smoothing capacitor 24 while decreasing. In this case, the energy of the inductor 21 is discharged. This switching operation is repeatedly performed in predetermined switching cycles so that the output voltage Vo is output from the smoothing capacitor 24. The output voltage Vo increases with an increase in the proportion of the on-time with respect to the switching cycle (the proportion is referred to hereinafter as a duty ratio δ). The output voltage Vo is represented by Vo=Vin/(1−δ). The control circuit 10 controls the main switch 22 using the switch signal whose duty ratio δ has been adjusted, to stabilize the output voltage Vo against fluctuations of input/output conditions.
In FIG. 4, the reference character 4 indicates a current detecting circuit including a reference transistor 41 of the same type as that of the main switch 22, a constant current source 42, and a comparator 40. The comparator 40 compares a reference voltage which is generated by the reference transistor 41 and the constant current source 42, with a connection point voltage between the main switch 22 and the inductor 21, and outputs the result as a current limit signal to the control circuit 10. Here, the reference transistor 41 is a transistor whose gate terminal is connected to a bias voltage Vdd and is always on. The control circuit 10 forcedly turns off the main switch 22 when determining, based on the input current limit signal, that the inductor current is going to exceed the limit current value.
The comparative input voltage Vm and the reference input voltage Vc of the comparator 40 are represented by:Vm=ILX×RON1  (1)Vc=RON4×IR  (2)where RON1 is the on-resistance value of the main switch 22, RON4 is the on-resistance of the reference transistor 41, IR is the current value of the constant current source 42, and ILX is the value of the inductor current.
The current detecting circuit 4 limits the current when Vm>Vc. Therefore, according to expressions (1) and (2), the limit current value ILM is represented by:ILM=RON4/RON1×IR  (3)
When the main switch 22 is of the same type as that of the reference transistor 41, and the reference transistor 41 has an on-resistance value which is M times as great as that of the main switch 22, RON4/RON1=M, resulting in:ILM=M×IR  (4)
The on-resistance ratio M can be set with high accuracy, and in addition, initial variations, of the on-resistance of each of the main switch 22 and the reference transistor 41, power supply fluctuations, temperature shifts and the like can be canceled.